Defect structure of NiO and rates and mechanisms of formation from atomic oxygen and nickel

Abstract

The oxidation of nickel by atomic oxygen at pressures from 6×10−3 to 0.33 Torr between 1050 and 1250 K has been investigated. In these ranges, the oxidation was found to follow the parabolic rate law, viz.kp = 1.14×10−5 exp(−13410/T)g2 cm−4sec−1 for films of greater than 1 μm thickness and was pressure-independent. The activation enthalpy for the oxidation reaction was 27±3 kcal mole−1. Of a number of possible mechanisms and defect structures considered, it was shown that, based on reaction activation enthalpies, impurity effects, pressure independence, and magnitudes of the rates, the most likely was a saturated surface defect model for atomic oxidation. A possible model judged somewhat less likely was one having equilibrium concentrations of doubly ionized cationic defects rate-controlling in both atomic and molecular oxygen. From comparisons of the appropriate processes, the following enthalpy values were derived: ΔH* (Ni diffusion in NiO) = 26.5 ± 8 kcal mole−1 and ΔHf0 (doubly ionized cation vacancies in NiO from atomic oxygen) = − 2.1 ± 6.0 kcal mole−1. The recombination coefficient of atomic oxygen on oxidized nickel was determined to be 0.14 ± 0.06 in the temperature range 985 to 1100 K.